1. Field of Application
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine. In particular, the invention relates to an engine exhaust gas purification apparatus in which a NOx catalyst in the exhaust system can be regenerated by temporarily establishing a fuel-rich condition of the exhaust gas.
2. Description of Related Art
In recent years, with increasing importance being placed on environmental protection, it has become essential to achieve effective purification of the exhaust gases emitted from the engines of motor vehicles.
In particular in the case of a diesel engine, it is necessary to effectively remove nitrous oxides (NOx) from the exhaust gas. Generally, this is performed by means of a LNT (lean NOx trap) installed at a position along the exhaust gas flow path, which utilizes a catalyst to adsorb and reduce (i.e., deoxidize) the NOx.
Such a LNT traps (adsorbs) the NOx while the diesel engine is operating in a fuel-lean exhaust gas condition i.e., a normal condition in which the exhaust gas has a high concentration of oxygen. When the exhaust gas subsequently is changed from the lean condition to a rich (i.e., fuel-rich) condition, with a low concentration of oxygen, the NOx that has been adsorbed by the LNT become reduced (deoxidized) by the fuel components in the exhaust gas, leaving nitrogen to be emitted from the exhaust system as a harmless gas. An LNT generally utilizes a material such as barium for adsorbing the NOx.
However with such a type of LNT, the agent used to adsorb the NOx combines with sulfur compounds in the fuel, resulting in a phenomenon referred to as sulfur poisoning. The performance of the LNT in adsorbing NOx is thereby lowered. In order to regenerate the LNT when this sulfur poisoning reaches an advanced state, it is necessary to temporarily establish a high-temperature (e.g., 650° C. or more), fuel-rich condition of the exhaust gas.
However fuel is wasted, when operating the engine to produce a fuel-rich exhaust gas in order to effect regeneration from sulfur poisoning. In particular, in the case of a diesel engine, the exhaust gas temperature is normally low. Thus, during each LNT regenerative operation, it is necessary to expend a large amount of fuel in order to maintain a sufficiently high temperature of exhaust gas until the regeneration procedure has been completed. It is therefore desirable to be able to accurately judge the point of completion of such a LNT regeneration procedure i.e., to judge the time point at which the regeneration procedure can be terminated. If that termination point can be accurately judged, then the amount of fuel consumed in each regeneration procedure can be minimized.
A method of judging such a termination point for sulfur poisoning regeneration is described for example in Japanese patent publication No. 2004-232576. With that method, applied to a system in which a fuel addition valve is controlled to inject fuel into the exhaust gas (i.e., for establishing a high-temperature fuel-rich atmosphere during sulfur poisoning regeneration), peak values of air/fuel ratio that are detected by an air/fuel ratio sensor (located downstream from the LNT catalyst) are applied in feedback control of the fuel addition valve. In that way, the volumetric rate at which fuel is injected by the valve becomes successively reduced as the sulfur poisoning regeneration process advances. When the rate falls below a predetermined value, the regeneration procedure is terminated.
However with the technique described in the above patent, it is difficult to directly measure the volumetric rate at which fuel is being injected by the fuel addition valve, so that it is necessary to estimate this based on command values of the volumetric rate. However due to deviations between the characteristics of respective fuel injection valve units, and deterioration of such a fuel injection valve over a period of time, etc., discrepancies will arise between the command values of volumetric rate and the actual rates at which fuel is added. Hence, the judgement accuracy will deteriorate.
If the time point for termination of sulfur poisoning regeneration procedure could be derived based on direct measurement, then greater accuracy of judgement could be achieved.